One word: Lard.
OK, I couldn't stick to one word. According to Diana Kennedy, lard has less cholesterol than butter. It is used widely here in Mexico, and I have finally started using it, too. In your tamales, it will give you a lighter, fluffier masa. The last time I made tamales, I actually used half lard and half butter because I like the butter flavor. Just don't use vegetable oil....you won't get the consistency you need. Good luck with your tamales...